1. Field of the Invention
The present invention generally relates to digital voice communications in a cellular system and, more particularly, to circuitry to control the power level of mobile units in a cellular telephone system. The power level control circuit according to the invention matches the performance of a matched diode pair yet is simpler in design.
2. Description of the Prior Art
The mobile telephone switching office (MTSO), or the base station under the control of the MTSO, sets the power level at which mobile/portable units (hereafter referred to collectively as mobile units) in a cellular telephone system can operate. In addition, most mobile units incorporate some form of closed loop control. Under current EIA standards and applicable regulations, the mobile transmitted power level assignment must be controlled by the MTSO. The MTSO is capable of monitoring the performance of the whole system and can increase or decrease the transmitted power level of the mobile units to render optimum performance. For a more detailed discussion of cellular communications, in general, and, in particular, power level control of the mobile units, the reader is referred to Mobile Cellular Telecommunications Systems by William C. Y. Lee, McGraw-Hill Book Co. (1989).
Because of the difficulty of maintaining constant RF power amplifier signal output level across a wide temperature range, variations in supply voltage, input signal level, frequency band and dynamic range, some current cellular telephones have a matched diode pair to temperature compensate power level control circuitry. An example is shown in U.S. Pat. No. 4,523,155 to Walczak et al. which discloses a temperature compensated automatic output control circuitry for power amplifiers which uses matched diode temperature compensation. While the circuitry using a matched diode pair is relatively simple, selecting matched pairs of diodes increases the cost of manufacture.
Other prior art systems use complicated and expensive schemes to avoid the need of temperature compensation. One example is shown in U.S. Pat. No. 4,952,886 to Braathen et al. which discloses an RF power control circuit wherein a sample of the RF output is amplitude modulated, then this signal is demodulated for level control thereby avoiding temperature sensitive direct RF detection. Thus, the problem of selecting matched pairs of diodes is traded for a more complex circuit, with its own contribution to the cost of the unit.